to the detector, and not via a reflection from some other
surface. The second step is to put an aperture between the
light and the detector that is smaller in diameter than the size
of the sample filter to be measured.
Next make an opaque shutter that can be put over the aperture
to make a zero measurement . With the light cut off, press
‘ZERO’
. Now remove the shutter without moving any of the
baffles or aperture. Press
‘SE
T
100%’
, which will be your
reference condition . The meter will read 100.0 at this time.
Now tape or mount your sample filter over the aperture
(again without changing the physical location of baffles or
aperture), and read the transmissio n directly on the ILT1700.
There are many spectral factors that must be considered
when making a transmissio n measurement. If you want to
know the transmissio n at one wavelength, it will be
necessary to use an interference filter to establish
monochromatic light before inserting the unknown filter. A
lamp plus monochromato r can be used to create a tunable
monochromatic source, for making a full spectral
transmission plot of the sample filter. A 100% reading must
be established at each wavelength since the source/
monochromato r combination will not be constant at each
wavelength. Often it is required to determine the
attenuation of an unknown filter material for a particular
source used in your system. In that case the only way to get
the same spectrum, is to duplicate the light used in the
system in question. (Reflectance and system throughput are
two more examples of uses for the percent mode. See
section 8.8 for details.)
3.2.5 Readout (Scientific Notation)
Most people with a technical background are completely
familiar with scientific notation, and will have no difficulty
interpreting the data on the display. For the remaining users
and as a review, we will briefly describe the notation and
how it relates to the readout.
There are two major parts to the readout: 1) The
mantissa is a three and a half (3 1/2) digit detailed portion of
the answer. It is designed to give at least 3 digits of
resolution, with the smallest increment less than one part in
200 for a readable answer better than 0.5%. 2) The second
part is the exponential portion, that tells which decade the
mantissa belongs in. In other words, the exponent is a
multiplier by powers of 10. If the exponent is zero, you
would multiply by ten raised to the zero power, which is the
same as multiplying by one. Likewise, an exponent of three
(3) would mean you multiply by one thousand (1000), and
so on. This system is necessary to handle the extremely
large change in light magnitude, and the tremendous
variation in measurement units. As an example, your eye
can see in an environment which can have a brightness
change of one million to one. If you couple this with the
variety of optical units, you can span more than 21 decades
of readout. Our instrument has been designed with the
ability to display magnitudes over 39 decades, just in case
you come up with a new variation. When in the
‘
FACT
OR’
mode, the display also reads out in scientific notation, which
is the same as presented on the calibration certificate.
There is one more mode of readout which is the percent
mode. By pressing
‘SE
T 100%
’
the system will read in
relative units, referenced to the magnitude existing when the
‘SE
T 100%
’
was pressed. The exponent will not be used in
this mode of operation, and auto ranging is turned off.
3.2.6 Darkroom Readings
The liquid crystal display relies upon reflectanc e of
available light therefore it will not readout in the dark. Our
solution was to locate
the ‘HOLD’
button on the right hand
side of the instrument, where it can be easily found in the
dark. With your fingers along the right side of the meter,
you can press the hold button when in total darkness. The
display will hold the reading present when the
‘HOLD’
button was pressed. This feature also applies to flash
integrations, where the final integral will be held on the
display until another function button is pressed.
3.3 Integration Measurements
The ILT1700 is capable of integrating energy in a flash of light
at microsecond speeds as well as int egrating for over eighteen
years, and everything in between.(with some limitations). The
chart below shows the charge conditions that can be measured.
The chart covers a very large dynamic range, w hich can be
misleading at a glance. On the lower right s ide of the chart the
instrument is limited by detector current. On the lower left side,
the limitation is due to charge insufficiency. The upper left
corner is restricted by peak current limitations of the detector.
The upper middle is limited by the con version rate of the
instrument. Finally the highest charges (upper centers) are
limited by maximum detector current again. Even with all of
those boundaries, the ILT1700 operates over more than 6
decades of charge ranges and more then a dozen decades of time
ranges. To relate this chart to optical measurements you must
multiply the energy you wish to measure, in units of either
Joules or Joules per centimeter square, by the sensitivity factor
of your detector. This product will give the charge that can b e
plotted on this chart. By the way, a Joule is equal to a
Watt*second, in case your units are broken out into the power
component.
12
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